Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Mon Aug 20 2018 Valid 12Z Thu Aug 23 2018 - 12Z Mon Aug 27 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Models and ensembles this morning continue to show very good agreement on the synoptic pattern evolution across the U.S. through next Monday. An upper-level trough is expected to lift out of the eastern U.S. late this week as an upper-level trough appears to become more amplified over the Pacific Northwest during the weekend into Monday. Discrepancies shown in the model solutions are relatively small even out to Day 7 next Monday. Therefore, a general model compromise was adopted to generate the WPC medium-range grid fields. For the sea-level pressure field, a 50-50 blend of the 00Z ECMWF with the 06Z GFS was used for Day 3, trending toward a 30%-70% blend of their ensemble means to their deterministic solutions from Days 5 through 7. This has kept good continuity with the previous WPC medium-range package. ...Weather Highlights and Hazards... A lingering stationary frontal boundary along the Southeast U.S. coast through most of the medium range period will result in scattered to numerous showers and thunderstorms with locally heavy rainfall possible. Areas of potentially heavy convection will also be possible from the northern/central plains to the Mississippi Valley and Upper Midwest through much of the period as a couple waves of low pressure traverse a frontal boundary across the region. Convection across the central U.S. may be confined more to the Midwest and Great Lakes by the weekend as upper-level heights begin to rise across the central U.S., increasing the likelihood of a capped environment farther south. Scattered afternoon/evening showers and storms are also expected to persist through the week across the Four Corners region, with monsoonal moisture remaining in place. Areas from the Mid-Mississippi Valley to the Mid-Atlantic will begin the medium range period with high temperatures 5 to 10 deg F below average before the upper trough lifts out. But as the upper ridge expands overhead, a gradual warming trend will follow as high temperatures 5 to 10 deg F above average are forecast to expand eastward from the the central U.S. toward the eastern U.S. during the weekend. Finally, height falls across the Northwest will lead to a cool trend with temperatures falling to more than 10 deg below average in the interior by Sunday into Monday. Kong/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml